For several decades, hard coatings have been applied to various types of articles to improve their resistance to wear in order to extend their operational lives. Hard coatings are commonly applied to cutting tools and wear-resistant components. Over the years, a great deal of effort has gone into the discovery of new hard coatings with improved properties.
For example, U.S. Pat. No. 7,060,345 to Fukui et al. teaches a hard coating which comprises one or more first layers composed of a lubricating compound selected from nitrides, carbides, carbonitrides, oxynitrides and carbonitrides of titanium silicon, and one or more second layers of a hard compound selected from the nitrides, carbides, and carbonitrides of a metal selected from the titanium, chromium, and a titanium chromium alloy. The first and second layers may be repeatedly deposited one upon the other to make up a hard coating made up of a plurality of alternating lubricating and hard layers.
U.S. Pat. No. 6,586,122 to Ishikawa et al. also teaches the use of a multilayer hard coating on a cutting tool. In this case, the multilayer hard coating includes a first layer comprising one or metallic elements selected from the group consisting of titanium, aluminum, and chromium, and one or more non-metallic elements selected from the group consisting of nitrogen, boron, carbon, and oxygen, and a second layer comprising silicon and one or more metallic elements selected from the group consisting of metallic elements of Group 4a, 5a, and 6a of the Periodic Table, and aluminum, and one or more nonmetallic elements selected from the group consisting of nitrogen, boron, carbon and oxygen. This second layer has a structure in which a silicon-rich phase is dispersed in a matrix phase containing a relatively small amount of silicon. The silicon-rich phase may be amorphous or crystalline silicon nitride or silicon and is formed by using a substrate temperature in the range of about 300 to 500° C. in conjunction with a substrate bias voltage that periodically changes between different levels of negative voltage or between positive and negative voltages.
Similarly, Japanese Unexamined Patent Publication 2000-334606 teaches a multilayer hard coating in which the hard coating comprises alternating layers of titanium aluminum nitride and titanium silicon nitride. The titanium aluminum nitride layers contain 40 to 75 atom percent aluminum. The titanium silicon nitride layers contain 10 to 60 atom percent silicon. The titanium silicon nitride layers contain independent phases of silicon and silicon nitride. A companion to this application, i.e., Japanese Unexamined Patent Publication 2000-334607, contains teachings of almost identical hard coatings, except it makes no mention of independent phases of silicon or silicon nitride. Instead, it describes the titanium silicon nitride layer as having a sodium chloride crystal structure, even though the hard coating compositions disclosed in this application are the same as those disclosed in its companion application. Comparing the two companion applications, it appears that the substrate-biasing voltage applied during the physical vapor deposition of the hard coatings is what is used to control the structure of the titanium silicon nitride coating. A low value negative bias voltage (−30 V) is used to create titanium silicon nitride layers which contain independent phases of silicon and silicon nitride, whereas a much higher negative bias voltage (−100 V) is used to create titanium silicon nitride layers having only the sodium chloride crystal structure. In both applications, the hard coatings are formed using substrate temperatures of no greater than 400° C.
Nonetheless, as production rates increase and efforts to provide manufacturing economic efficiencies intensify, there continues to be a need for longer-lasting, more wear resistant cutting tools and wear components. It is an object of the present invention to address that need.